In recent years, water-absorbent resins are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids.
Examples of the above water-absorbent resins include: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, or crosslinked products of these hydrolyzed products; and crosslinked polymers of cationic monomers.
Hitherto, it has been said that the above water-absorbent resins should have properties such as excellent absorption amount of liquid, absorption rate, gel strength, gel liquid permeability, and suction force to suck up water from a base material containing aqueous liquids, when they are in contact with aqueous liquids such as body fluids. Then, various water-absorbent resins (water-absorbing agents), combing at least two of these properties and displaying excellent performance (absorption properties) when they are used for sanitary materials (e.g. disposable diapers and sanitary napkins), has been proposed hitherto.
For example, as to a method for improving absorption properties (e.g. absorption capacity without load and absorption capacity under a load) of a water-absorbent resin in good balance, there is a known art in which the surface neighborhood of the water-absorbent resin is crosslinked, and then various methods have been disclosed.
Known examples thereof include methods in which the following materials are used: polyhydric alcohols (JP-A-180233/1983 and JP-A-016903/1986); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-189103/1984); glyoxal (JP-A-117393/1977); polyvalent metals (JP-A-136588/1976, JP-A-257235/1986 and JP-A-007745/1987); silane coupling agents (JP-A-211305/1986, JP-A-252212/1986, and JP-A-264006/1986); alkylene carbonates (DE 4020780); polyvalent heterocyclic carbonates (JP-A-315216/1999); oxazolidinones (WO 99/42494); polyvalent oxazolidinones (WO 99/43720); oxadines (WO 00/31153); and oxazoline compounds (JP-A-197818/2000).
And further, when the improvement of the absorption properties is carried out by the above crosslinking agents, there are also known methods in which additives (e.g. inert blending promoters, acid catalysts, and bases) are used in order to further improve the performance. That is to say, known examples of the methods (1) in which the inert blending promoters are used as the additives include methods in which the following materials are allowed to exist: inert inorganic powders (JP-A-163956/1985 and JP-A-255814/1985); water containing salts and/or hydroxides of polyvalent metals (JP-A-007745/1987); dihydric alcohols (JP-A-292004/1989); water together with ether compounds (JP-A-153903/1990); water-soluble polymers (JP-A-126730/1991); alkylene oxide adducts of monohydric alcohols, monovalent salts of organic acids or lactams (JP-B-074331/1994 and JP-A-033818/1995); monovalent metal salts (WO 98/49221); and cations (WO 00/53664 and WO 00/53644).
Furthermore, known examples of the methods (2) in which the acid catalysts are used as the additives include methods in which the following materials are allowed to exist: phosphoric acid (WO 94/15651); and inorganic acids or organic acids (JP-A-278225/1995).
In addition, known examples of the method (3) in which the bases are used as the additives include a method in which water-soluble alkaline compounds are allowed to exist (JP-A-298841/1994).
Because these additives, as used in the above methods (1), (2), and (3), exist together with the crosslinking agent, the balance of the absorption properties of the water-absorbing agent can also be improved to a certain extent in comparison with the case that the crosslinking agent exists alone. However, it was yet still difficult to say that the balance was sufficient.
Specifically, as to the additives (inert blending promoters) as used in the above method (1), the effect is caused by acting as blending promoters when the water-absorbent resin as used contains fine powders in a large amount. On the other hand, because of their existence, there were problems such that: almost no improvement of the absorption properties is observed, for example, because the absorptivity of the crosslinking agent to a water-absorbent resin powder is excessively lowered or because the crosslinking reaction is inhibited; and even if the improvement is achieved, it is necessary to increase the amount of the crosslinking agent as used, to prolong the reaction time, and to raise the reaction temperature.
As to the additives (acid catalysts) as used in the above method (2), the effect as a catalyst that promotes the reaction of the crosslinking agent can be expected. However, when the amount to obtain a certain effect is added, the pH of a crosslinking agent solution is extremely lowered and, especially in such as cases of water-absorbent resins of a partially neutralized type containing an acid group, the acidification of the surface is caused, so that the control of the absorptivity of the crosslinking agent is difficult. In addition, there are disadvantages in that: the acidification of the surface increases the adhesion between the water-absorbent resin particles, and it tends to form aggregates. As a result, there are problems such that: the desired crosslinking density of the water-absorbent resin particle surface layer cannot be obtained; and it is difficult to obtain what displays satisfactory performance.
As to the above method (3), there is disclosed the surface-crosslinking which is carried out by combination of the additives (bases) with compounds having at least two functional groups easily reactable with a carboxyl group (e.g. multivalent metal salts, polyepoxy compounds, polyaziridinyl compounds, and polyisocyanate compounds), and the improvement of the gel strength and absorption capacity under a comparatively light load (20 g/cm2) are intended. However, in the method as described in JP-A-298841/1994, the improvement of the blendability of the surface-crosslinking agent or the properties of the water-absorbent resin is still insufficient. Particularly, it was difficult to improve an SFC and an absorption capacity (AAP) under a heavy load (4.83 kPa, about 50 g/cm2) (both are mentioned below.), as requested in recent years.
In addition, examples of typical water-absorbent resins include acrylic water-absorbent resins comprised of crosslinked products of partially neutralized acrylic acid salts, from the viewpoint of high properties and costs. Then, as to production processes for such acrylic water-absorbent resins, the following two methods are generally carried out: a method which involves polymerizing acrylic acid and its salt as neutralized to a predetermined neutralization ratio beforehand (hereinafter, referred to as neutralization polymerization method); and a method which involves polymerizing unneutralized or low neutralized acrylic acid; and thereafter post-neutralizing the resultant polymer gel (hereinafter, referred to as acid-type polymerization method).
In comparison with the neutralization polymerization method, the latter acid-type polymerization method tends to obtain a water-absorbent resin having a high absorption capacity and a low extractable content. However, it takes plenty of time to uniformly neutralize a crosslinked hydrogel polymer after polymerization, and further the neutralization is technically very difficult, and there are cases where the neutralization ratios of individual particles of the resultant water-absorbent resin powder are non-uniform among those particles. JP-A-010173/1998 (EP 0882502 A1) discloses that: in this case, although the water-absorbent resin of the acid-type polymerization method has a high absorption capacity and a low extractable content, and even if the surface-crosslinking treatment is carried out, the performance of the water-absorbing agent is not sufficiently obtained.
That is to say, when water-absorbent resins are surface-crosslinked, there has hitherto been a case where the necessary surface-crosslinking treatment is different depending upon the difference of the neutralization ratio, therefore the desired performance of the water-absorbing agent is not obtained at other neutralization ratio even if the optimum crosslinking treatment for a certain neutralization ratio is carried out. Particularly, when the neutralization ratios of the individual particles of the water-absorbent resin powder are various, there has hitherto been a case where the desired performance of the water-absorbing agent is not be obtained.
As is mentioned above, the surface-crosslinking treatment cannot be uniformly carried out by the prior arts. As a result, the balance of various properties (such as a CRC, AAP, and SFC as mentioned below, particularly SFC) of the water-absorbent resin as obtained went bad, and further the scatter of the SFC was caused. Therefore, for example, the scatter of properties of diapers among lots and the great difference of properties even in one diaper were caused.